1. Field of the Invention
The present invention relates to a technical field of an optical pickup used for optically reproducing and recording information on a recording medium such as an optical disk in an optical disk player, an optical disk recorder, and so on, and an information reproducing apparatus having the optical pickup.
2. Related Art
Such an optical pickup comprises a light source, a photo detector, an optical path splitting element such as a beam splitter which reflects a light beam emitted from the light source in such a manner as to guide the light beam to a recording medium and transmits reflected light from the recording medium in such a manner as to guide the reflected light to the photo detector, and an object lens which is opposed to an information recording surface of the recording medium. In general, the optical pickup comprises a quarter-wave plate between the object lens and the optical path splitting element to rotate the polarization plane of light which is incident and reflected on the recording medium, that is, to change the polarization state of a light beam. The “polarization state” of the present invention indicates circular polarization, linear polarization, or elliptic polarization. In the case of linear polarization, the polarization state also indicates its direction.
On the other hand, in the case of such an optical pickup, in a light spot formed on the information recording surface by the radiation of a light beam, a region having intensity higher than predetermined intensity (i.e., a so-called far-field region in the light spot) is generally shaped like an ellipse. The attaching angle or the like of optical components is set so that the major axis direction of the ellipse is orthogonal to or in parallel with the direction of a recording track composed of information pits on the information recording surface.
It has been known that during the reproduction of an optical disk, the optical pickup configured thus reduces a signal-to-noise ratio of a reproduction signal due to birefringence on the information recording surface. The birefringence splits light constituting reflected light into two in a transparent layer such as a protective layer of the optical disk. The birefringence is caused by a manufacturing process of the optical disk using a stamper or the like. The birefringence occurs because the distribution of a refractive index in the information recording surface is varied between the inner periphery and the outer periphery of the optical disk. Thus, a variation in jitter amount, that is, a variation in the time base direction of a reproduction signal goes out of a permissible range, so that it becomes difficult to correctly reproduce information.
Hence, various techniques are conventionally proposed. In these techniques, a reproduction signal is generated which is relatively larger than noise (disk noise) on a surface of an optical disk including a transparent layer for generating birefringence and thus the reproduction signal is increased in signal-to-noise ratio. For example,the following optical pickup is proposed: a light beam of linear polarization that is emitted by a semiconductor laser is caused to pass through a quarter-wave plate and thus the light beam is converted into a light beam of circular polarization light, so that the reproducing characteristic of an optical disk is improved without being affected by the direction of birefringence.
Further, the following optical pickup is proposed: a light beam of linear polarization is emitted from an arrangement having a rotated semiconductor laser, the light is incident along a polarization direction at 45° with respect to the direction of a recording track formed of information pits on an information recording surface, and thus the reproducing characteristic of an optical disk is improved.
However, as described above, an optical pickup for generating a light beam of circular polarization needs to additionally comprise the quarter-wave plate to generate a light beam of circular polarization, so that the cost is increased and the apparatus becomes larger or complicated.
Further, according to the research conducted by the inventor, circular polarization can be formed by the phase control of existing optical components without the necessity for adding a quarter-wave plate. However, it is necessary to satisfy the condition of an optical path configuration in an optical pickup, that is, the condition that an incident angle from a half mirror to a rising mirror (reflection mirror) is set at 45° with respect to the parallel line of the rising mirror. Thus, a technical problem arises that it becomes difficult or practically impossible to arrange constituent elements so as to satisfy the above condition in a miniaturized optical pickup.
As described above, when a light beam of linear polarization is emitted from an arrangement having a rotated semiconductor laser and the light is incident along a polarization direction at 45° with respect to the direction of a recording track formed of an information pit on an information recording surface, an angle of 45° with respect to the recording track is formed by the major axis direction of a far-field region in a light spot formed by the light beam. Thus, a technical problem arises that a signal for reproducing an information pit, that is, an RF signal is reduced as compared with the case where the major axis direction of the far-field region is caused to intersect the recording track direction.